Diffractive optical elements (DOE) for focusing light and performing other functions by utilizing a light diffraction phenomenon to change the direction in which light is propagated have drawn attention in recent years. In a diffractive optical element, prominent portions and groove portions are repeatedly formed on the surface, and the machining precision thereof has a significant effect on the optical characteristics of the element. For this reason, numerous studies have been conducted relating to methods for machining a diffractive optical element, and various machining methods have been proposed (see, for example, Japanese Laid-open Patent Publication No. 11-197902, Japanese Laid-open Patent Publication No. 10-138004, and International Patent Publication Pamphlet No. 2003/055826).